It was a fitting tribute to Carl Sagan’s imagination, optimism, and starry-eyed wonder. On Monday, which would have been Sagan’s 75th birthday, the Planetary Society announced that it is pushing ahead with a plan for experimental spacecraft that will ride on sunbeams, powered by solar sails. The first small craft will be sent into orbit in 2010, if all goes well, and will be followed by two others, which may venture farther. Sagan was a founder of the Planetary Society and a big booster of solar sail plans.

Solar sail technology, which has not yet been tested in space, relies on the tiny impacts created by the light particles streaming from the sun as they hit a reflective surface. The force on a solar sail is gentle, if not feeble, but unlike a rocket, which fires for a few minutes at most, it is constant. Over days and years a big enough sail, say a mile on a side, could reach speeds of hundreds of thousands of miles an hour, fast enough to traverse the solar system in 5 years. Riding the beam from a powerful laser, a sail could even make the journey to another star system in 100 years, that is to say, a human lifespan [The New York Times].

The spacecraft that is scheduled for orbit in 2010, the LightSail-1, has been made possible by an anonymous donation to the Planetary Society. The recent donation reinvigorates the Society’s solar sail hopes, which were dashed in 2005 when the Russian Volna rocket carrying its first solar sail prototype, Cosmos 1, failed to reach orbit…. In addition to the Cosmos 1 disappointment, NASA’s NanoSail-D attempt was lost in the third failed flight of SpaceX’s Falcon 1 rocket in 2008 [SPACE.com].

Imagine what these things could do with focused light, like beamed energy, where you can get a beam that is 1,000 suns — 1,000 times the same amount of solar radiation per m² at earths distance from the sun. The sail would use carbon nanotubes that can take the heat.

Once they proof test this out beamed energy will be the next important test. A way to get more solar radiation is to get close enough to the sun without over heating before the sail is unfurled, about as close as Mercury 8 to 10 suns…